This invention relates to the synthesis of porous crystalline materials in fluoride media.
Recently, a number of porous crystalline materials have been produced using a synthesis regime that employs hydrofluoric acid as the mineralizer instead of the more conventional alkali hydroxides. Thus, the synthesis of chabazite, ITQ-3, ITQ-4, ITQ-9, SSZ-23, SSZ-24, SSZ-31, CIT-5 and ZSM-12 in fluoride media is disclosed in the article entitled xe2x80x9cSynthesis of All-Silica and High-Silica Molecular Sieves in Fluoride Mediaxe2x80x9d by Camblor, M. A.; Villaescusa, L. A.; Diaz-Cabanas, M. J. in Topics in Catalysis 1999, 9, 59-76. However, these syntheses usually produce pure silica or silica/germania polymorphs that are catalytically inactive. In certain cases, the syntheses may be induced to produce boron or aluminum substituted materials, but generally, adding these elements diverts the synthesis to other, undesirable, materials. As is well known, the boron or aluminum is needed to produce materials that have the ion exchange capacity and acidity needed to make active catalysts so the materials produced by these new synthetic methods are often not catalytically active or are contaminated with active materials of an undesired structure.
Current synthesis regimes employing hydrofluoric acid mineralizers normally involve mixing a structure directing agent, usually a quaternary ammonium hydroxide, and tetraethylorthosilicate (TEOS) at high dilution in water and then slowly evaporating water and ethanol at low temperature until the H2O/SiO2 ratio is reduced to a desired low level. The resulting, often solid, mixture is then mixed by hand with hydrofluoric acid such that the molar ratio of HF/base is at or below 1.1:1.0 and the resulting mixture crystallized under hydrothermal conditions in normal zeolite synthesis apparatus. It is critical that the evaporation of water and ethanol be carried out at low temperature, usually 20-40xc2x0 C., so the evaporation is tedious. Because the rate of evaporation is a function of surface area/volume, it is impractical to scale the synthesis to more than a few grams and hence it is unsuitable for commercial production.
The present invention seeks to address two important needs related to molecular sieves produced in fluoride media. It allows such materials to be synthesized with reduced impurities in the presence of boron or aluminum and it allows the synthesis to be scaled from a few grams to commercial quantities.
The present invention is directed to a process for synthesizing a porous crystalline material method which comprises the steps of:
(i) preparing a mixture capable of forming said material, said mixture comprising sources of water, an oxide of a tetravalent element Y, an oxide of a trivalent element X, wherein X is selected from the group consisting of aluminum, boron, gallium, iron, and indium, an organonitrogen cation RNm+ capable of directing the synthesis of said material, and fluoride ions, wherein the molar ratio of fluoride ions to organonitrogen cations RNm+ in said mixture is greater than 1.15 m but less than 2.5 m;
(ii) maintaining said mixture under crystallization conditions until crystals of said material are formed; and
(iii) recovering said crystalline material from step (ii).
Preferably, the molar ratio of fluoride ions to organonitrogen cations RNm+ in said mixture is from about 1.25 m to about 1.8 m. Preferably, said tetravalent element Y includes silicon. Preferably, said mixture contains an inorganic source of silica.